1. Field of the Invention
The present invention relates to a color liquid crystal display device, particularly to a color liquid crystal display device provided with a color filter fabricated by employing the principle of thermal dye transfer technology.
2. Description of the Prior Art
The liquid crystal display is classified into a simple matrix type and an active matrix type according to the pixel selection method.
The simple matrix type liquid crystal display has two sets of electrodes arranged crosswise to each other with the liquid crystal such as STN in between, and forms pixels at cross sections of the electrodes.
The active matrix type liquid crystal display, on the other hand, has nonlinear devices (switching devices) provided in one-to-one correspondence to individual matrix-arranged pixel electrodes. Because the liquid crystal at each pixel is theoretically activated at all times (duty ratio of 1:1), the active type has good contrast compared with the simple matrix type that employs a time-division drive system. The active type therefore is becoming a technology essential to a color liquid crystal display in particular. Thin-film transistors (TFT) are known as typical switching devices.
The basic structure of a color liquid crystal display comprises a substrate on which a common pixel electrode is formed, another substrate on which individual electrodes or switching devices are formed, and a liquid crystal composition sealed between these substrates, with the first substrate provided with color filters.
FIG. 15 is an exploded perspective view showing components of a liquid crystal module that uses a TFT type liquid crystal display as an example of the color liquid crystal display that applies this invention. Denoted SHD is a framelike shield case (metal frame) made of a metal plate; LCW a liquid crystal display window as a display area in the shield case; PNL a liquid crystal display panel; SPB a light dispersing plate; MFR an intermediate frame; PCB3 an inverter circuit; BL a backlight; BLS a backlight support; and LCA a lower case. These components are stacked in a vertical relationship shown in the figure to form a liquid crystal module MDL.
The module MDL is held firmly together by claws CL and hooks FK provided to the shield case SHD.
The intermediate frame MFR is rectangular to form an opening corresponding to the display window LCW, and its frame portion is provided with the light dispersing plate SPB, the backlight support BLS, depressed and raised portions conforming to shapes and thicknesses of various circuit components, and openings for dissipating heat.
The lower case LCA also serves as a reflector for backlight and is formed with reflection ridges RM opposing the fluorescent lamps BL to ensure efficient reflection.
The backlight may be produced by other than the back illumination type shown in the figure. For example, a side illumination type may be employed in which a light source is arranged on the side of the liquid crystal display panel PNL. In this case, a planar light source structure is used which consists mainly of a light conducting body located under the light dispersing plate SPB.
FIGS. 14(a) to 14(c) show sectional views of a conventional active-matrix color liquid crystal display device which represents an example of a color liquid crystal display device of the type to which the present invention is directed. As shown in the drawing, a bottom transparent glass substrate SUB1 and a top transparent glass substrate SUB2 are provided on respective sides of a liquid crystal layer LC. Moreover, a thin-film transistor TFT1 comprising a gate electrode GT, gate insulating films AOF and GI, a semiconductor layer AS, and source and drain electrodes SD1 and SD2, and a transparent conductive electrode ITO1 are formed on the transparent glass substrate SUB1 side, and moreover a transparent protective overcoat PSV1 and a lower orientation film ORI1 are formed in order. A light-shielding film BM serving as a black matrix, color filters FIL(R), FIL(G), and FIL (B), a transparent protective overcoat PSV2, a common transparent conductive electrode ITO2(COM), and an upper orientation film ORI2 are formed in order on the inside surface (liquid crystal LC side) of the top transparent glass substrate SUB2. Silicon oxide films SIO formed through dipping are provided on both sides of the transparent glass substrates SUB1 and SUB2. A sealing pattern SL is formed along the perimeters of the substrates SUB1 and SUB2 and between the transparent glass substrates SUB1 and SUB2 so as to seal the liquid crystal LC except for a liquid crystal sealing hole. The sealing material is made of, for example, an epoxy resin. The common transparent conductive electrode ITO2(COM) on the top transparent glass substrate SUB2 side is connected to a lead wire INT formed on the bottom transparent glass substrate SUB1 side by a silver paste material AGP at least at one portion of the four corners of a panel in this embodiment. The lead wire INT is formed using the same fabrication process as that of a gate terminal, which is not illustrated, and a drain terminal DTM.
The orientation films ORI1 and ORI2, the transparent conductive electrode ITO1, and the common transparent conductive electrode ITO2 are formed inside the sealing pattern SL. Polarizing plates POL1 and POL2 are formed on the outside surfaces of the bottom transparent glass substrate SUB1 and the top transparent glass substrate SUB2, respectively. The liquid crystal LC is sealed in a space defined by the sealing pattern SL between the upper orientation film ORI1 and the lower orientation film ORI2, both for setting the orientation of the liquid crystal molecules. The lower orientation film ORI1 is formed on the transparent protective overcoat PSV1.
The liquid crystal display device of the present invention is fabricated by forming various layers on the bottom transparent glass substrate SUB1 side and the top transparent glass substrate SUB2 side separately, forming the sealing pattern SL on the substrate SUB2 side, joining the bottom transparent glass substrate SUB1 and the top transparent glass substrate SUB2 together, pouring liquid crystal LC through an opening of the sealing material SL, closing the sealing hole with epoxy resin or the like, and cutting the top and bottom substrates.
Therefore, the i-type semiconductor layer AS of the thin-film transistor TFT1 is sandwiched by the light-shielding film BM thereabove and the slightly large gate electrode GT thereunder so that it is shielded from external natural light or backlight. The light-shielding film BM is formed like a grid around each pixel element (so-called black matrix) and the effective display area of each pixel element is defined by the grid. Therefore, the contour of each pixel element is made clear by the light-shielding film BM and the contrast is improved. That is, the light-shielding film BM has two functions: shielding of the i-type semiconductor layer AS from light and a black matrix.
The light-shielding film BM, as shown in FIGS. 14(a) and 14(c), is formed like a frame also at the periphery, and its pattern is formed continuously with a pattern of a matrix section having dot-like openings. The light-shielding film BM at the periphery is extended to the outside of the sealing section SL to prevent leakage light, such as reflected light attributive to an apparatus mounted with this crystal display device, such as a personal computer, from entering the matrix section. Moreover, the shielding film BM is formed 0.3 to 1.0 mm inside from the edge of the substrate SUB2 so as to avoid the cut area of the substrate SUB2.
An active-matrix liquid crystal display device using a thin-film transistor is disclosed, for example, in Japanese Patent Laid-Open No. 309921/1988 and "Jocho koseiwo saiyoshita 12.5-type akutibu matorikusu hoshiki kara ekisho disupurei", Nikkei Electronic, pp. 193-210, issued by Nikkei McGraw-Hill Inc., Dec. 15, 1986. The disclosed embodiment is of an active matrix type. However, the light-shielding black matrix pattern BM is not always necessary for a color liquid crystal display device using an inexpensive super-twisted nematic (STN) liquid crystal or a twisted nematic (TN) liquid crystal because thin-film transistors are not used. The present invention relates to an ordinary color liquid crystal display device. Therefore, also in this case, the present invention can be applied to processes other than the black matrix process.
Conventionally, to form a color filter for a liquid crystal display device, the dyeing method, pigment dispersion method, electrodeposition method, and printing method have been used so far, which mainly involve a photolithography technique. FIG. 16 shows a fabrication method according to the pigment dispersion method using a photolithography process which is most popular among these methods. The BM forming process includes a process for forming a metallic chromium film or the like on a black matrix and thereafter forming a pattern by photoetching, and a process for adding a black colorant to a photosensitive resist, coating it and thereafter forming a pattern by photolithography. In the process for forming pixel elements of a color filter array, photosensitive resist containing pigment particles is coated and thereafter exposure and development are repeated for red (R), green (G), and blue (B) to form a pattern.
FIG. 17 shows a structural sectional view of a general color filter formed by the fabrication method shown in FIG. 16. In FIG. 17, symbol ITO2 denotes a transparent conductive electrode formed on the color filter surface, PSV2 denotes a transparent protective overcoat formed on colored layers, in which FIL(R), FIL(G), and FIL(B) denote colored pixel elements, SUB2 denotes a glass substrate, and BM denotes a black matrix. A silicon oxide film SIO may not be formed depending on the purpose or the material of the transparent substrate SUB2.
As shown in FIG. 17, the color filter normally has a structure in which the protective overcoat layer PSV2 is formed on the colored layers FIL(R), FIL(G), and FIL(B) whose pattern area is separated like a mosaic, a vertical or horizontal stripe for each pixel element or each color, and moreover, the transparent conductive electrode ITO2 is formed on the layer PSV2. By forming the above color filter structure, it is possible to improve the heat resistance of the color filter to a temperature close to 200.degree. C. due to vapor deposition and sputtering in forming the transparent conductive electrode ITO2, and the thermal treatment in the module process after the vapor deposition and sputtering up to a level in which the color filter can practically be used, and thereby a color filter with a high color reproducibility is formed.
Methods for simultaneously performing formation of three primary colors have been studied for practical use in order to reduce the fabrication cost and improve the productivity. The thermal dye transfer technology is one of these methods. The thermal dye transfer technology applied to color printing used for color copying machines and video printers will be described below. FIG. 18(a) shows the fabrication process and FIG. 18(b) shows a typical example of the fabrication system. The resist applying process is a process for forming image receiving paper. In the case of this example, a dye image-receiving layer is provided by a coating process on a base paper and a protective layer for protecting against abnormal image transfer is formed on the dye image-receiving layer. The protective layer prevents ink from thermally diffusing up to an undesired area at the time of thermal dye transfer. In the present invention, however, no protective layer for protecting against abnormal image transfer is used because a protective area against dye diffusion is formed which prevents dye from thermally diffusing up to an undesired area as a result of the structure of the color filter. Then, the image receiving paper is colored in the thermal dye transfer process. A thermal dye transfer film 2 is a film wherein a stick resistant layer is formed on one side of a base film made of polyethylene terephthalate or the like and a colorant is formed on the other side. The color of a colorant is realized through the color mixing method by using three primary colors of yellow (Y), (C), and magenta (M). The thermal dye transfer technology generally uses two methods according to the types of coloring material. By the first method a coloring material is produced by mixing a colorant such as pigment with, for example, wax. Pigment is transferred to an object to be colored together with the wax through heating. The advantage of using the colorant is that plain paper can be used as the object to be colored. The second method uses a base film coated with a mixture of a sublimatable dye and a binder resin as an ink layer 3. In this case, the object transferred when heated is the dye. The second method uses the sublimation of the dye due to heating. A dye receiving layer requires a transparent macromolecular dyeing resin film layer.
Since the present invention involves a problem concerning the fact that a transparent substrate, such as a glass substrate is used instead of base paper, the method using wax becomes unsuitable from the point of view of the heat resistant characteristic, and so the second method is used as a better choice in accordance with the present invention. Though the present invention uses a thermal head as a heat-generating body 4, it is also possible to use a laser.
An example in which the second method of the thermal dye transfer technology is applied to the color filter of a color liquid crystal display device is shown in the following documents: D. J. Harison and M. C. Olidfield, "The Use of Thermal Dye Transfer Technology for the Fabrication of Color Filter Arrays", Proceeding of the 9th International Congress on Advances in Non-Impact Printing Technologies, pp. 382-384 (1993) and U.S. Pat. No. 5,166,126.
Though the colored layer of a conventional color filter is fabricated by a method mainly using a photolithography process, as shown in FIG. 16, the method is the primary cause of an increase in cost because it requires a long fabrication process. Moreover, because the photolithography process always includes light exposure processes, the method requires a mask with a higher accuracy as a pattern becomes finer. Furthermore, a development process using liquid chemicals is indispensable in order to pattern a macromolecular layer exposed to light. Furthermore, to form three colored layers of red (R), green (G), and blue (B), a problem arises in that the above exposure and development processes must be repeated at least three times. Furthermore, there are problems that, when they are actually used for a liquid crystal device, the liquid crystal-driving transparent electrodes on a protective overcoat are not properly formed since the three colored layers have different thicknesses because they are separately formed, or a film thickness variation of the liquid crystal LC between pixel elements increases when assembling a liquid crystal device, while maintaining a certain gap with an opposed electrode substrate. For a STN-type liquid crystal device, in particular, it is necessary to further decrease the film thickness variation of the liquid crystal LC compared to a TN-type liquid crystal device in order to improve the response speed and the view angle characteristic. Therefore, if there is film thickness variation of the liquid crystal LC in a certain plane, defective color irregularity occurs at the portions where the thickness is uneven and this is very disadvantageous for stabilization of the optical characteristics.
It is possible to simplify the coloring process by using a conventional thermal dye transfer technology using a sublimatable dye as a fabrication process as shown in FIGS. 18(a) and 18(b). However, because dyeing is performed by dye sublimation employing heat, there is a problem that the dye thermally diffuses from a colored pattern when it is subjected to high temperature and this causes color fading or tone change.